In general terms, in a switching mode power supply an input voltage is modulated by a switch and the modulated waveform is passed through an inductance, rectified and filtered to provide an output of a controlled value. Such switching mode power supplies are very well known and examples of basic topologies are boost convertors, buck convertors and buck boost convertors.
In order to control the switching and therefore the output, it can be helpful to know the average current, or at least the peak current which is a fair approximation of the average current, passing through the inductor.
The current passing through the inductor can be measured directly by including a current sensing resistor in series with the inductor and measuring the voltage drop across that resistor. A problem with this solution, however, is that there are undesirable energy losses in the resistor. Also one cannot normally use a current sensing transformer in series with the inductor because this would become saturated by the DC component of the inductor current.
An alternative solution is to determine the currents flowing through the switch and the rectifier and to use those determinations to derive the current flowing through the inductor. This is possible because, irrespective of the relative positioning of the inductor, rectifier and switch, one has in effect three branches in the resulting electronic network, one containing the inductor, one containing the switch and one containing the rectifier, and all three branches have a common point so that the current in the branch containing the inductor will always be the combination of the currents through the switch and rectifier.
In order to determine the currents passing through the switch and rectifier, one could include current sensing resistors in series with each but this suffers from the problem noted above of undesirable energy losses and there would normally not be any advantage in using two resistors in place of a single resistor in series with the inductor. Therefore current sensing transformers have been included in series with these branches. This has cost and space disadvantages, however, since two are required and they are relatively large components.
The invention has therefore been made with these points in mind and it is an object of the invention to provide a compromise between these various factors.